The invention pertains to filled spacers for filled cell electrodialysis apparatus, commonly called EDI or electrodeionization apparatus. EDI apparatus is commonly implemented as a rectangular or cylindrical “stack” of spaced-apart selectively permeable membranes, or as a spiral-wound module of such membranes, in which the structural geometry of the stack elements and selective membrane permeabilities define alternating dilute and concentrate cells positioned between two or more electrodes. A first liquid feed is flowed through the dilute cells and, in passing through the cells, gives up its undesired mineral content to exchange resin in the cells. The captured mineral species are then transported through selectively permeable anion or cation exchange membranes into adjacent concentrate cells, where the species are flushed by a separate second fluid flow. External plumbing and suitable power switching circuitry may be provided to allow some constructions to be operated as reversing electrodeionization (“EDIR”) units, which are operated by periodically reversing the polarity of the electrodes and interchanging the flows through the dilute and concentrate cells.
A variety of constructions for the spacer or frame elements of a filled cell electrodialysis apparatus (including EDI, EDIR, and electrodiaresis units) are known, for example, as described in U.S. Pat. Nos. 2,923,674; 4,465,573; 4,632,745; 5,120,416; 5,292,422 and 6,123,823 among others. Each of these patents is hereby incorporated herein by reference. In these prior art spacer constructions, each liquid flow path is typically an open or unobstructed region of a cell bounded by membranes and spacer walls, and filled, at least in part, with functional material such as ion exchange beads, felt, fibers or the like. Constructions typically involve a spacer frame that surrounds the cell providing an enclosed chamber and also supporting the edges of consecutive selectively permeable membranes so that the membranes are spaced apart by the nominal spacer or frame thickness. Ribs, posts or other elements, either integral with the spacer frame or added during assembly, may support the membranes apart at intermediate positions and define sub-compartments or narrow, parallel flow paths within the area of the overall frame/spacer.
These known constructions and methods of maintaining separation between the membranes of filled cell electrodialysis cells depend upon the mechanical strength of the membrane to assure that the membranes support themselves across the limited span between ribs or supports. Thus, for example, several manufacturers have employed solid ridges or ribs spaced no more than a few inches apart to separate the EDI flow cells into parallel long, narrow, flow channels between the two broad area membranes. The width, thickness and mechanical modulus or strength of the material used in the spacers or frames has also imposed limits on the width of fluid flow paths and the maximum operating pressures and temperatures of these stacks. When long rectilinear flow compartments are employed, the flow distribution in the sub-compartments or flow channels defined by such supported spans of membrane may be subject to edge effects at interfaces between membrane and spacer side walls, creating regions of low or dead flow, and also causing channeling or bypassing by the liquid flow, all of which can result in less than optimal fluid treatment or sub-optimal stack performance. The cells are typically filled with a swellable body of ion exchange beads that must be maintained in physical contact with the exchange membranes to effect proper ion transport. The proper filling of a dilute cell with a suitable quantity of an appropriate mixture of ion exchange beads to maintain stable membrane spacing and support, assure conductive contact of the resin, and provide an appropriately low flow impedance is not a straightforward task but rather one that may involve considerable experimentation.
In the concentrate cells of such EDI apparatus, particularly in stacks not designed for reversal operation (EDIR), the flow cells typically have no ion exchange filling and the membrane spacer/frame may be generally of thinner construction. For these unfilled cells, a screen or mesh material has often been used to support the membranes apart and to assure a non-occluded liquid flow path between the membranes. The screen thus serves as a dimensional spacer in the thickness direction transverse to the plane of liquid flow. Such a concentrate cell screen-type spacer may consist of one or more layers of a sheet-like mesh material, and is typically configured to provide a relatively low flow impedance (e.g., a sufficiently unobstructed flow cross-section), while the fibers constitute small local obstructions that enhance flow turbulence at the membrane surfaces thus causing this flow to efficiently carry off the species that have been removed from the first liquid flow and transported across the membrane by ionic conduction. Such operation maximizes one factor involved in the transfer efficiency. The mesh may be a relatively limp gauze-like material, a thin non-woven heat-extruded mesh, or a somewhat extensible and/or compressible fabric having a tricot-like fiber geometry.
In addition to the above-described constructions of bead-filled dilute cells and screen-containing concentrate cells, various other constructions have been considered. Some researchers have proposed, for example, that the spacer frame elements, or even the screen/mesh itself be formed of ion exchange material, that a cell filling include ion-exchange felt or textile, or that the ion exchange membranes incorporate ion exchange fibrils that project from the membrane surface into a fluid flow path, but various cost and practical construction factors have limited commercial implementation of such proposals.
Most of the membrane, resin and spacer materials used in these devices are polymeric, and both the resin and the membrane materials are typically swellable in use. Thus, whatever specific constructions may be employed, the provision of thin, resin-holding flow cells defined between many sets of parallel ion exchange membranes that are separated by spacer frames, poses a complex problem of hydraulics, chemical engineering and structural design. The construction must be stacked and bolted closed to form a hydraulically sealed vessel comprised of many well-defined thin flow cells, and it must simultaneously provide good flow and effective ion-exchange treatment conditions without giving rise to excessive membrane stress or damage, or bulging, cracking or leakage of the spacer frames, and without impairing the operation or long term performance of the stack as a whole.